This invention relates to circuits having semi-conductor diodes and more particularly to a system and method of lowering the effective forward voltage drop of semi-conductor diodes.
Semi-conductor diodes are used extensively in half wave and full wave rectifier circuits. It is well known that semi-conductor diodes conduct current in a forward direction but substantially no current flows when the diode is reverse bias. Further, in the forward direction current only flows after the applied voltage exceeds a predetermined voltage and this voltage is known as the forward voltage drop. The value of the forward voltage drop is dependent on the material from which the semi-conductor diode is fabricated. The forward voltage drop of a silicon diode, for example, is typically 0.7 volts. It is, of course, known to manufacture diodes by other techniques which may result in a lower forward voltage drop. For example, Schottky diodes have a low forward voltage drop but the process necessary to fabricate Schottky diodes is not always compatible with the process used in manufacturing the integrated circuit incorporating the semiconductor diode.
There are several applications wherein the input voltage to the circuit is limited and the forward voltage drop of a standard diode may adversely affect the efficiency of the circuit such as, for example, a rectifier circuit.
In the particular application for which a low forward voltage drop is advantageous is an RF-tag circuit. RF tag circuits are used to communicate various forms of information from and to remote locations. These can be active RF tags which automatically transmit information or semi-passive RF tags which are activated or interrogated by another device.
The present invention applies in particular to systems wherein the RF tag is interrogated and in response returns information such as location, temperature, humidity, pressure etc. In such systems the rectifier in the input circuit rectifies incoming data from an external source and extracts from the incoming data the supply voltage (vdd and vss) for the integrated circuit in the RF tag unit.
Obviously, the forward voltage drop can affect adversely is the efficiency of the rectifier circuit. Therefore, there is a need for a system and method to effectively lower the forward voltage drop of diodes used in low voltage environment.
The present invention provides a simple circuit that can reduce forward voltage drop of diodes and provide thereby increased efficiency and output voltage of a rectifier circuit.
A current reference controls secondary effects of the circuit. The circuit of the present invention is of particular value in integrated circuit fabrication where standard application processes are employed and Schottky diodes are not available.
The invention is also well suited for low voltage and low power applications where the forward voltage of a diode limits the dynamic range.
Therefore, in accordance with a first aspect of the present invention there is provided a circuit for lowering the effective forward voltage drop-of a semi-conductor diode comprising: a first metal oxide semi-conductor (MOS) device connected in parallel with the semi-conductor diode, the MOS device having a set threshold voltage; and a second MOS a device for establishing a bias voltage for the first MOS device, the bias voltage being less than the threshold voltage by a small value.
In accordance with a second aspect of the present invention there is provided a full wave rectifier circuit comprising first and second complementary diodes, each of the diodes having an associated circuit for lowering its effective forward voltage drop, the associated circuit comprising: a first metal oxide semi-conductor (MOS) device connected in parallel with each of the semi-conductor diodes, the MOS device having a set threshold voltage; and a second MOS device for establishing a bias voltage for the first MOS device, the bias voltage being less than the threshold voltage by a small value.
In accordance with a further aspect of the present invention there is provided a method of reducing the effective voltage drop of a semi-conductor diode comprising: connecting a first metal oxide semi-conductor (MOS) device in parallel with the diode, the first MOS device having a predetermined threshold voltage; and generating a bias voltage for the first MOS device, the bias voltage being less than the threshold voltage; whereby input voltage applied to the diode is added to the bias voltage to turn on the MOS device, thereby bypassing the diode.